The invention relates to a reactor having a contact tube bundle through whose space surrounding the contact tubes a heat-exchange medium circuit is passed, and to the use of the reactor for carrying out oxidation reactions.
The conventional design of reactors of the generic type consists of a generally cylindrical tank in which a bundle, i.e. a multiplicity, of contact tubes is accommodated, usually in a vertical arrangement. These contact tubes, which may contain supported catalysts, are attached with their ends in tube bases in a sealing. manner and lead into a hood connected to the tank at the upper end and a hood connected to the tank at the lower end. The reaction mixture flowing through the contact tubes is fed in and led out via these hoods. A heat-exchange medium circuit passes through the space surrounding the contact tubes in order to equalize the heat balance, in particular in the case of highly exothermic reactions.
For economic reasons, reactors having the highest possible number of contact tubes are employed, the number of contact tubes accommodated frequently being in the range from 15000 to 30000 (cf. DE-A-44 31 949).
Regarding the heat-exchange medium circuit, it is known to implement a substantially homogeneous temperature distribution of the heat-exchange medium in each horizontal section through the reactor in order that wherever possible all the contact tubes participate equally in the reaction events (for example DE-B-16 01 162). Smoothing of the temperature distribution is effected by heat supply or dissipation via outer ring lines installed at the reactor ends and having a multiplicity of jacket apertures, as described, for example, in DE-B-34 09 159.
A further improvement in heat transfer is achieved by installation of baffle plates which leave a passage cross section alternately in the reactor center and at the reactor edge. Such an arrangement is particularly suitable for tube bundles in an annular arrangement with a free central space and is disclosed, for example, in GB-B-31 01 75.
In large reactors having a number of contact tubes in the abovementioned region of from about 15000 to 30000 and which are additionally equipped with baffle plates, the pressure drop of the heat-exchange medium is in comparative terms very large. For this reason, the eutectic salt melt comprising potassium nitrate and sodium nitrite which is frequently used to dissipate the heat liberated during oxidation reactions and has a viscosity similar to that of water at a use temperature of from to about 350 to 400xc2x0 C. must be pumped into a reactor of the above size at a feed height of about 4 to 5 m in order to overcome the pressure drop.
In large reactors of this type, the pump system is advantageously located between the upper and lower ring line, with the heat-exchange medium being fed into the lower region of the reactor, for example via a ring line.
If, in large reactors of this type, the salt melt were to be pumped directly into the upper part of the reactor or the upper ring line, the requisite feed height of 4 to 5 m would require a technically unfavorable and fault-susceptible pump system, inter alia due to complex pump-shaft seals, longer pump shafts, and greater heat introduction through the pump shaft into the lower motor bearing. Furthermore, the abovementioned feed height would require a high-level salt-melt compensation vessel, which is undesired for safety reasons.
Supply of heat-exchange medium to the upper end of the reactor, i.e. in concurrent with the reaction mixture, likewise fed into the contact tubes at the upper end of the reactor, is, as is known, advantageous for reaction implementation (cf. DE-A-44 31 449).
The cocurrent implementation has advantages over the counter-current procedure, such as higher throughputs, lower catalyst hot-spot temperatures, a welcome increase in the heat-exchange medium temperature toward the end reaction in the contact tubes, good temperature uniformity of the heat-exchange medium over the reactor cross section, i.e. good horizontal temperature layering, clear operating states above the height of the contact tube space owing to the lack of back-coupling through the heat-exchange medium.
However, cocurrent transport of reaction mixture and heat-exchange medium, as described in DE-A44 31 449 or shown in DE-A-22 01 528, FIG. 1, comes up against the abovementioned problems regarding the pump system if the heat-exchange medium is fed to the upper region of the reactor, for example directly via an upper ring line, and discharged from the lower region of the reactor, for example directly via a ring line.
It is an object of the present invention to provide a reactor which does not have these disadvantages regarding the pump system. The pump system should not be modified compared with the design with feed of the heat-exchange medium into the lower ring line in the lower region of the reactor and discharged from the upper ring line which has proven successful for large reactors having a multiplicity of contact tubes, for example up to 40000, in particular from 15000 to 30000 contact tubes; nevertheless, the heat-exchange medium should flow around the contact tubes in cocurrent with the reaction mixture fed through the contact tubes.
We have found that this object is achieved by a reactor having a contact tube bundle through whose space surrounding the contact tubes a heat-exchange medium circuit is passed, with ring lines at both ends of the reactor with jacket apertures for the supply and discharge of a heat-exchange medium by means of one or more pumps, if desired with the heat-exchange medium or a substream of the heat-exchange medium being passed through one or more external heat exchangers, in which case the heat-exchange medium is fed to the lower ring line and fed back to the pump(s) via the upper ring line, and with baffle plates which leave a passage cross section alternately in the reactor center and the reactor edge, wherein the upper and lower ring lines are each divided into an inner and outer ring line by means of a cylindrical partition wall, and the heat-exchange medium is fed through the outer lower ring line, via a region outside the reactor to the inner upper ring line, via the latter""s jacket apertures to the space surrounding the contact tubes, via jacket apertures into the inner lower ring line and subsequently via a region outside the reactor is discharged via the outer upper ring line.
It has been found that the space between the upper and lower ring lines can be utilized to divert the heat-exchange medium, allowing the advantage of cocurrent transport of heat-exchange medium and reaction mixture to be combined with the proven pump arrangement with feed of the heat-exchange medium to the lower ring line.
To this end, the invention provides that a cylindrical partition wall is arranged in the upper ring line and in the lower ring line, separating each of these lines into an inner and outer ring line. The heat-exchange medium is then fed to the outer lower ring line, which is connected to the inner upper ring line via the region between the upper and lower ring lines, from here is fed in a known manner via jacket apertures into the space surrounding the contact tubes, with a meander-like flow being formed in a known manner via baffle plates. The heat-exchange medium leaves the space surrounding the contact tubes, in the lower part of the reactor, in a known manner via jacket apertures and enters the lower inner ring line. This is in turn connected to the upper outer ring line via the region between the upper and lower ring lines.
The region between the upper and lower ring lines is advantageously closed by a cylinder envelope, forming a hollow cylinder, which is divided, by radial partition walls perpendicular to the reactor base, into chambers whose dividing walls to the ring lines leave alternately inner and outer circular ring sections, where, in plan view, an open circular ring section is always arranged above a closed circular ring section, and vice versa. The chambers thus always experience flow, from bottom to top, alternately by heat-exchange medium coming from the pump(s) and heat-exchange medium coming from the reactor space.
The number of chambers is in principle unlimited, but expediently a number of from 12 to 96, preferably from 24 to 48, can be provided, so that from 12 to 24 chambers (corresponding to from 3 to 6 chambers per quarter) are alternately available for the transport (redirection) of the heat-exchange medium to the inlet in the upper region of the reactor space surrounding the contact tubes and to the outlet from the lower region of same.
The cylindrical partition walls which separate each of the upper and lower ring lines into an inner and outer ring line may in principle have any diameter between the external and internal diameters of the ring lines. However, the diameter of the cylindrical partition walls is preferably less than or equal to the arithmetic mean of the outer and inner diameters of the ring lines.
In a preferred embodiment, a bypass chamber with jacket aperture to the reactor space and a regulating plate is arranged in the region of the outer upper ring line in each case in at least some of the chambers through which heat-exchange medium flows and is discharged to the pump(s), the position of the regulating plate being adjustable in the direction of the longitudinal axis of the reactor via an actuating drive and a drive spindle. In this configuration, an adjustable sub-stream of the heat-exchange medium coming from the reactor space can be taken off as early as the central height of the reactor, meaning that only the remaining volume of the heat-exchange medium flows through the lower part of the reactor space surrounding the contact tubes only experiences. This embodiment is optimized with respect to decreasing heat evolution in the lower part of the contact tubes. In addition, a reduction in the pressure drop is achieved, which permits reduced pump output and thus increased economic efficiency.
The reactor is not restricted with respect to the type of heat-exchange medium, which can be used equally to dissipate heat, i.e. for the performance of exothermic reactions, and for the supply of heat to the reaction mixture flowing through the contact tubes, i.e. for the performance of endothermic reactions.
The reactor is particularly suitable for the performance of oxidation reactions, in particular for the preparation of phthalic anhydride, maleic anhydride, glyoxal, (meth)acrolein and (meth)acrylic acid.